Establishment

Oilseed rape is a particularly difficult crop to establish, for instance its small seeds are highly susceptible to dessication having once imbibed water. Establishment has been further complicated by the ban on burning cereal straw residues, thus making seedbed preparation even more difficult as the straw must be either removed or incorporated into the soil (McWilliam et al, 1995). It is widely accepted that good establishment is a key factor for successful cropping. Growers seed rates are about 6-7kg/ha and are increased in poor conditions, although this increase can cause problems later in the season from an increased risk of lodging due to high plant populations. Therefore some growers sow at seed rates as low as 2.5kg/ha, but at optimum drilling dates and into optimal seedbed conditions in order to maximise plant size into the winter.

Traditionally, growers have viewed poor initial growth and low plant numbers with poor returns, but the evidence now suggests that good strong, vigorous plants coming through the winter at populations as low as 10-12 plants/m² are at an economic threshold to produce a good crop (assuming plants are evenly spaced). This has been illustrated by Mendham et al (1981) and Spink (1992) who found significant yield advantages from crops with poor initial establishment and more open canopies. Daniels et al (1982) also identified the flexibility of rapeseed plants to compensate for poor establishment and pest damage to produce equally high yields as those from higher plant populations. This compensatory effect in response to differing plant populations is mainly due to its ability to alter the branching and hierarchical structure of the plant (ie. indeterminate). Subsequently, plants grown at high density have fewer branches and thinner stems (Daniels et al 1982).